1. Field of the Invention
The present invention relates to ceramic materials which are mainly composed, having a crystalline phase, of aluminum titanate and mullite and having high heat resistance, and excellent thermal shock resistance and thermal shock durability.
2. Related Art Statement
In industrial technology to solve new problems occurring with technological development, demands for high performance have not been attained with respect to the materials to be used for such industrial technology.
As to ceramic materials having excellent corrosion resistance, those further possessing heat resistance and thermal shock resistance having been called for. The thermal shock resistance of ceramics is influenced not only by characteristics such as the coefficient of thermal expansion, thermal conductivity, strength, modulus of elasticity, Poisson ratio, etc, of the material, but also by the size and the profile of a product made therefrom as well as heating and cooling states, that is, heat transfer speed. Among these characteristics influencing the thermal shock resistance, particularly the coefficient of thermal expansion has a large contributory factor. Particularly, when the heat transfer speed is large, it is known that the thermal shock resistance is greatly influenced only by the coefficient of thermal expansion. Thus, the development of materials having excellent thermal shock resistance as well as a low coefficient of thermal expansion has been strongly demanded. At the same time, it has also been desired that the heat resistance of the material be high.
On the other hand, the coefficient of thermal expansion of an aluminum titanate crystal, which constitutes the aluminum titanate ceramics, is largely different depending upon the crystal axes. When the thermal stress exceeds a limit of the strength of the constituting crystals or grain boundaries, microcracks are formed inside the grains or the grain boundaries. Consequently, the strength of the ceramic is decreased and it is likely to crack or break when used as a product. Therefore, development of materials having high strength and low coefficient of thermal expansion have been earnestly desired.
As the materials satisfying such a desire, there has been developed low expansion type ceramics mainly composed of magnesium oxide-aluminum oxide-titanium oxide-iron oxide-silicon oxide, as a crystalline phase (see U.S. Pat. No. 4,316,965 and European patent application laid-open No. 37,868).
Thereafter, there has been developed a material which contains aluminum titanate and mullite as a main crystalline phase and uses iron oxide and/or rare earth metal oxides as a sintering additive in such an amount as not to diminish the low coefficient of thermal expansion of aluminum titanate (European patent application laid-open No. 133,021 and U.S. Pat. No. 4,483,944).
The above-mentioned materials are excellent ceramic materials which possess high refractoriness and high thermal shock resistance as well as high mechanical strength and pay due consideration upon the stability of crystals of aluminum titanate under a continuously high temperature condition.
However, when a honeycomb structural type catalytic converter having a number of fine through holes as disclosed in the above publications is produced and used, the dimension gradually becomes larger than the original dimension due to an employed thermal cycling and will not restore to the original dimension. Further, such a converter is sooner or later fractured due to the dimensional growth. That is, the above materials have a problem in that are poor in durability when they undergo a thermal cycling.
Nevertheless, the conventional improvement of the aluminum titanate-mullite base material gives priority on the stability of the crystal of aluminum titanate under the continuously high temperature condition. Therefore, there still remains a problem that it is an extremely difficult technique to obtain a material which has refractoriness, heat resistance and thermal shock resistance as well as high mechanical strength and increased durability against the abovementioned thermal cycling.